1) Field of the Invention
The invention relates to aqueous dispersions of plastics which have a reduced formaldehyde content and contain one or more peroxy compounds from the group comprising peroxides, perborates, percarbonates, persulfates and perphosphates for reduction of the content of free formaldehyde.
2) Background of the Invention
Aqueous dispersions which are self-crosslinking if appropriate and are based on synthetic resins are used to bond nonwovens, which are mainly used in the medical or hygiene sector. The base polymer usually consists of homo- or copolymers of acrylic acid esters or methacrylic acid esters, vinyl acetate homopolymers or vinyl acetate-ethylene copolymers, as well as polymers based on styrene-butadiene or vinyl chloride, if appropriate with a content of self-crosslinking comonomers. The crosslinking which proceeds during the drying operation on self-crosslinking polymers causes an increase in wet and dry strength of the nonwovens under mechanical stress and improves the resistance of the nonwovens to cleaning with water and solvents.
The crosslinking agents employed in practice are mainly monomers containing N-methylol groups, such as N-methylol derivatives of unsaturated organic acid amides (N-methylol-acrylamide) or esters thereof (N-(isobutoxymethyl)acrylamide). N-Methylol compounds split off formaldehyde in an aqueous medium. When these compounds are used as crosslinking agents in an aqueous medium, free formaldehyde is split off above all during drying, because of the condensation reaction. The free formaldehyde thus formed is still contained in the nonwoven even after intensive drying.
The redox catalysts employed during the polymerization, for example Bruggolit or Rongalit (alkali metal formaldehyde sulfoxylates), which also release formaldehyde in an aqueous medium, are another source of free formaldehyde in aqueous synthetic resin dispersions.
Because of the toxicological objections to formaldehyde, which have been discussed for a relatively long time, and the stipulation that only limited formaldehyde concentrations are permitted in the finished nonwoven for nonwovens in the medical and hygiene sector, there exists the need to provide dispersions of plastics having a reduced formaldehyde content as binders for nonwovens.
DE-A1 33 28 456 (EP-A1 143 175) claims formaldehyde-free, crosslinkable polymer systems having crosslinking components based on N-methylol amide and/or N-methylol ether-amide groups. The reduction in formaldehyde content is achieved by addition of a formaldehyde acceptor based on cyclic ureas, such as, for example, ethyleneurea, which bonds the free formaldehyde formed. The disadvantage of this procedure is that specifically the wet strength values of the bonded nonwovens are reduced by the addition of water-soluble organic substances, and formaldehyde is still present, although in bonded form, and can be released, for example, under exposure to heat.
A path analogous to that of DE-A1 33 28 456 with the disadvantages just mentioned is taken in EP-B1 80 635. In this case, urea is added to the dispersion as a formaldehyde scavenger.
The present invention therefore has the objective of drastically reducing the formaldehyde content in aqueous dispersions of plastics based on self-crosslinking polymers or non-crosslinking polymers, without adversely influencing the strength of substrates bonded with these dispersions and avoiding renewed release of formaldehyde during processing.